(1) Field of the Invention
The present invention relates to a method for conditioning the CdTe layer of CdTe thin-film solar cells without the use of CdCl2.
(2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The production of superstrate configuration CdTe thin-film solar cells according to the state of the art involves the application of a transparent front contact layer (TCO, i.e. transparent conductive oxide) onto a substrate (preferably glass). The front contact layer may also be a layer stack consisting of several different layers. A layer of pure or modified CdS (cadmium sulfide) is deposited onto that. Modified CdS (cadmium sulfide) is henceforth understood to be CdS with doping, variations in the crystal shape or in grain sizes, or as a mixture of CdS with other substances. A layer of CdTe (cadmium telluride) is applied on top of the CdS layer. The back contact layer, or back layer sequence, is in turn deposited onto the CdTe layer.
An issue arising from this is the difficulty of applying a metallic contact layer onto the CdTe, since this would create a rectifying Schottky contact. What is desired, however, is the creation of an ohmic contact. An interface layer sequence is therefore used in the prior art. Its object is to realize the approximation of energy levels of the individual layer materials so that an ohmic contact is created, with a metallic layer being the preferred top back contact layer.
After the CdTe is applied during the production process in the prior art, there is usually an activation of CdTe follows by means of CdCl2 and heating. To this end, a CdCl2 layer is applied to the CdTe layer using (preferably wet-chemical or using CVD or PVD) methods according to the state of the art. Afterwards, the CdCl2 is left to react with the CdS/CdTe layer stack at a raised temperature (usually about 380° C. to 430° C.). Reaction time is about 15 minutes to 45 minutes. Here the CdCl2 acts as fluxing agent and supports a recrystallization of the CdTe layer.
After activation, any surplus CdCl2 is rinsed off from the layer surface using deionized water. A problem arising from the activation process according to the state of the art is that CdCl2 is an easily water-soluble, toxic chemical and highly hazardous for the environment. It has been shown to have carcinogenic and mutagenic properties, and it is toxic for reproduction. These properties require special care in handling when processing the CdCl2 in the production process. Furthermore, the generated contaminated waste water requires costly cleaning or waste disposal.
Thus the object consists in replacing the CdCl2 used in the process with a safer, and easier to handle substance, yet at the same time largely maintaining the process parameters for layer activation.